The subject matter disclosed herein relates to multi-energy X-ray imaging systems and, more particularly, to systems and methods for producing increased energy separation in X-ray spectra applied by such systems.
In modern medicine, medical professionals routinely desire to conduct patient imaging examinations to assess the internal tissue of a patient in a non-invasive manner. For typical single-energy computed tomography (CT) imaging, the resulting X-ray images are largely a representation of the average density of each analyzed voxel based upon the attenuation of X-rays between the X-ray source and the X-ray detector by a patient or object. However, for multi-energy X-ray imaging, a greater amount of imaging data may be gleaned for each voxel. For example, in a dual-energy X-ray imaging system, X-rays with two different spectra are applied to the patient or object; the higher energy X-ray photons are generally attenuated substantially less by patient tissue than the lower energy X-ray photons. In order to reconstruct images from multi-energy CT projection data, the underlying physical effects of X-ray interaction with matter, namely, the Compton scattering effects and photoelectric effects, are incorporated in a process known as basis material decomposition (MD), which is known in the art.
During multi-energy CT data acquisition, a multi-energy X-ray source may be used to apply X-rays having different energy spectra and may be capable of quickly switching from emitting X-rays having one spectrum to emitting X-rays having a different spectrum. Such sources are typically called fast-switching kVp (peak source voltage) sources because the input voltage to the source is switched quickly between high and low potentials to enable acquisition of closely positioned projection data (low-energy projection data and high-energy projection data)—both spatially and temporally. However, the rapid kVp switching requirements from a single X-ray source limits the ability to employ dynamic X-ray beam filtration schemes between the high- and low-energy projection data acquisitions, i.e. rapidly switching a filter out of and into the X-ray beam during low-energy and high-energy acquisitions, respectively. Dynamic filtering schemes are employed to selectively filter the high-energy X-ray spectra to improve the mean energy separation between the low-energy and high-energy spectra. The mean energy of a spectrum is the energy level of an average photon in the spectrum; it is computed by summing all energies in a given X-ray spectrum after weighting each energy by the percentage of photons at that specific energy. Thus, without dynamic filtration, there is significant spectral overlap in the low-energy and high-energy projection data acquisitions—limiting the mean energy separation between the projection data acquisitions. Energy separation is desirable in multi-energy images because it enhances material decomposition methods, which improves the clinical usefulness of the reconstructed images. As known in the art, multi-energy images comprise basis material images, monochromatic images (images reconstructed as if the applied X-ray spectrum consisted of a single energy), or images reconstructed directly from an applied energy spectrum. Accordingly, there exists a need for systems that enable multi-energy X-ray imaging with fast-switching sources that apply energy spectra with improved mean energy separation.